1. Field of the Invention
The present invention relates to a signal input sheet used to input a signal at a desired position on the surface of a sheet in response to the pressure of the position.
2. Background Art
Various on-line real-time character recognition devices have been known which recognize information while describing the information to input handwritten information (see the official gazelle of Japanese Patent Laid-Open No. 56-124973). As handwritten input devices used for these devices, various sheet-like input devices have been known which utilize the distortion of the surface of a sheet due to the writing pressure when information is described on the surface of the sheet to obtain a position signal according to the variation of writing order.
These sheet-like input devices are designed so that a conductive surface is positioned in an isolated state on a basic resistance surface which is a detecting base plate, and the conductive surface contacts the base resistance surface according to a writing pressure to form a variable electric circuit to sequentially obtain position signals. The isolated structure of the conductive surface for the basic resistance Surface includes a pressure sensitive conductive rubber in which a conductive material is discontinuously distributed in the rubber so that the conductive material continuously keeps conductivity when a writing pressure is applied and a mesh sheet in which a sheet is with holes like meshes is set between the basic resistance surface and the conductive surface so that the conductive surface contacts the basic resistance surface to obtain conductivity through the meshes when a writing pressure is applied (see the official gazette of Japanese Patent Publication No. 59-7992).
This applicant proposes a signal input sheet to be put on a base with a basic resistance surface at its surface and having a conductive layer at the bottom facing the basic resistance surface as disclosed in the official gazette of Japanese Patent Publication No. 3-48529. As shown in FIG. 2, the signal input sheet 1 is put on a base made by forming a conductive paste layer "b" on the surface of an etching substrate "a" (the surface of the layer "b" serves as a basic resistance surface 4 to be mentioned later), in which a conductive layer 11 is formed at the back face of a sheet 10 made of a soft material and a predetermined surface of the layer is divided into an electrical point contact surface (symbol 111 to be mentioned later) and a non-contact section and an insulating material made of insulating ink 12 containing a material contacting with the conductive layer 11 is formed on the conductive layer 11 of the non-contact section.
Moreover, the applicant proposes a signal input sheet having a conductive shield layer so that the static electricity of a human body will not affect devices as disclosed in the official gazette of Japanese Patent Laid-Open No. 61-43332. As shown in FIG. 2, the signal input sheet 1 has a structure in which a cover film 15 made of a soft material, an adhesive layer (non-adhesive paste) 131, and a shield layer 13 are laminated on the sheet 10 in order from the top.
The existing signal input sheet, as described above, has a cover film made of a soft material such as a urethane film on its surface to improve the feeling and writing condition of the surface. However, because the cover film slightly harder than the sheet is put on the soft sheet, there is a problem that the sheet is inferior in its durability under the state in which local deformation of the signal input sheet is repeated due to repetition of writing.
Some signal input sheets use a hard surface film made of a hard material such as a PET film (polyethylene terephthalate film) or hard surface material made of a glass thin plate for their surface. In this case, however, the input feeling is bad, a writing utensil may slip, and the feeling and writing condition are not good. Moreover, though these sheets withstand a writing pressure equal to or lower than the set load, permanent deformation is left when a writing pressure larger than the set load is applied and the feeling and writing condition are greatly impaired. Those using the glass thin plate may be cracked. Therefore, a dedicated writing utensil (designed so that the writing pressure does not exceed a certain value) is normally attached to these signal input sheets. However, the dedicated writing utensil is restricted in the writing condition or the like. Therefore, it is impossible to cover every condition.